Relatively little progress has been made in reaching the target of safe and effective non-invasive transdermal delivery of formulations for macromolecules, including peptides and proteins. Barriers to developing transdermal formulations for proteins, peptides and other large and small molecules include poor intrinsic permeability, cellular enzymatic degradation and chemical instability. Pharmaceutical approaches to address these barriers that have been successful with traditional small, organic drug molecules have not readily translated into effective peptide and protein formulations. The ability of molecules to permeate the skin effectively appears to be related to molecular size, lipid solubility and peptide protein ionization. Molecules less than 1000 daltons appear to cross the skin barriers rapidly. As molecular size increases, the permeability of the molecule decreases rapidly. Lipid soluble compounds are more permeable than non-lipid soluble molecules. Maximum absorption occurs when molecules are un-ionized or neutral in electrical charges. Charged molecules, therefore, present the biggest challenges to absorption through the skin.
Some enhancers, especially those related to bile salts, and some protein solubilizing agents are extremely potent in transporting the molecules effectively across the tight junctions and skin. Several approaches have been utilized to improve the transport of the bile salt-based delivery systems, including the use of protease inhibitors and various polymer matrices. Other attempts to deliver large molecules using single bile acids or enhancing agents in combination with protease inhibitors and biodegradable polymeric materials similarly failed to achieve therapeutic levels of proteinic drugs in the patient. Single enhancing agents fail to loosen tight cellular junctions for the time needed to permit passage of large molecules through the skin membranes without further degradation.
Various transmission systems have been proposed in connection with the delivery of small molecules such as local anesthetic compounds. U.S. Pat. No. 5,013,545 to Blackmon et. al. discloses aqueous gel-containing topical medications comprising high concentrations of alcohol, water and topically effective amounts of a pharmaceutical active such as hydrocortisone, diphenhydramine hydrochloride, lidocaine or miconazole nitrate in a gel matrix primarily consisting of water-soluble carboxyvinyl polymers. A gel clarifying agent may be optionally added for aesthetic reasons.
U.S. Pat. No. 4,937,078 to Mezie et. al. discloses the incorporation of certain concentrations of topical anesthetic actives into liposomes which are of a substantially greater size than nano particles. U.S. Pat. No. 5,081,158 to Pomerantz discloses the use of medicated protective films as a carrier for topical anesthetics. The films are comprised of hydroxypropyl cellulose (HPC) and an esterification agent which renders the HPC soluble in a non-volatile solvent such as ethanol, isopropanol or methanol. Medicinal compounds such as benzocaine and a variety of other topical anesthetics, antibiotics and steroids are incorporated which, when applied to the skin, result in situ formed medicated films from which the actives are released to provide a sustained supply of the medicine at the treatment site.
U.S. Pat. No. 5,002,974 to Geria discloses a topical anesthetic and skin moisturizing composition comprising any one of a number of topical anesthetics, including pramoxine, in an oil-in-water emulsion including a dissolved surface active agent. The composition is asserted to provide an aesthetically pleasing analgesic skin care product. The emulsion not only provides relief from the pain associated with irritated skin but is asserted to soften and moisturize the skin with an oily coating. U.S. Pat. No. 4,493,591 to Fourman et al discloses skin care cosmetic formulations comprised of a cellulosic polymer/solvent system capable of dispersing thin, substantive films upon the skin. Such films may serve as a carrier for sun blocking agents and insect repellents and also serve to prevent water loss form the skin surface to the environment.
Finally, U.S. Pat. No. 4,389,418 to Burton et. al., in a more general and traditional sense, discloses the use of hydrocarbons such as petrolatum, paraffin wax and ozokerite and other emollients as skin moisturizing materials. These function by covering the skin with a hydrophobic occlusive film which prevents water loss from the skin to the environment.